Techniques

The contrast mechanism of TOF MRA is based on the inflow effect. Fully relaxed blood entering the measured volume behaves as an endogenous contrast agent, by producing a bright signal. The bright depiction of flowing blood, however, requires the use of flow rephasing techniques (GMR) in order to overcome the effects of spin dephasing due to transverse magnetization. TOF MRA using GRE sequences has several advantages: Firstly, GRE sequences are not affected by the wash-out phenomenon that diminishes the signal of fast flowing blood when using SE techniques. Secondly, GRE techniques permit the use of short repetition times (TR < 40 msec), which are needed to efficiently saturate stationary tissue. Thirdly, echo times can be kept short (TE < 5 msec), thus further reducing spin de-phasing. Generally, it is advisable to apply in-phase echo times in order to avoid opposed-phase effects at the vessel walls that would impair the depiction of small vessels. Finally, GRE techniques are characterized by short acquisition times which are important when acquiring volume (3D) datasets.

TOF techniques can be divided into three groups (Fig. 10):

• sequential 2D multi-slice method,

M, maximum signal partial signal loss total signal loss

Fig. 9a-c. a If all spins in a voxel have the same velocity the signal is maximal. bIf there is velocity distribution, loss of phase coherence occurs resulting in decreased signal intensity. c If phase dispersion is total, the signal intensity is zero

Fig. 10. TOF-techniques: 2D multi-slice, 3D singleslab, 3D multi-slab

Scan Direction Motsa

Fig. 10. TOF-techniques: 2D multi-slice, 3D singleslab, 3D multi-slab

With 2 D techniques, the vessel is imaged by sequentially scanning multiple thin slices. This method has two advantages in comparison to the interleaved multi-slice technique: Firstly, very short TR times can be used which boost the inflow effect, and secondly, partially saturated blood is hindered from flowing from one slice to another. The method guarantees sufficient inflow enhancement even in vessels with a very slow flow and produces constant vessel-background contrast in the covered region, because each slice is an entry slice [2].

Problems arise with 2D TOF MRA if the vessels to be imaged do not flow in a perpendicular direction to the imaging plane. If the vessels run partly inside the slice (i.e. in-plane) or return to the slice in a looped form, then signal loss may occur due to the partial saturation of flow.

In order to achieve a sufficient signal-to-noise ratio, a slice thickness of at least 2-3 mm is necessary. However, this results in reduced spatial resolution and increased spin dephasing due to the larger voxel size required. 2D slices do not have rectangular RF profiles and therefore exhibit signal variations at the edges that can lead to steplike artifacts in maximum intensity projection (MIP) reconstructions. However, this effect can be largely overcome by overlapping the slices.

In vessels exhibiting very pulsatile flow, the extent of inflow enhancement varies during the heart cycle. The periodic change of inflow enhancement generates ghost images of the vessels. Saturation of blood spins can occur due to slow flow in the diastolic phase. The use of ECG triggered 2D TOF sequences may overcome many of these problems by confining data acquisition to the phase of maximum inflow. Thus, blood signal is enhanced and ghost artifacts are eliminated. By synchronizing data acquisition with the heart cycle, the vessel is mapped in each slice with equal intensity. Unfortunately, a drawback of this approach is the prolonged acquisition time.

In 3D TOF MRA, the entire imaging volume, usually 30 - 60 mm thick, is excited simultaneously and then partitioned into thin slices by an additional phase encoding gradient along the slice-select direction [3]. 3D TOF MRA has the advantage of high spatial resolution together with high signal-to-noise ratio, thereby facilitating the improved depiction of particularly small vessel structures. The technique allows slices of less than 1 mm thickness and isotropic voxels to be acquired easily.

One major problem of the 3D technique, however, is the progressive saturation that occurs when blood flowing through the volume is subjected to repeated RF pulses. As a result, the signal intensity decreases continuously in the direction of flow. The extent of saturation depends on the length of time in which the blood stays inside the volume. In slow flow vessels, signal begins to diminish when only a short distance has been covered. Conversely, in faster flowing blood the signal remains visible for a greater distance. Consequently, the maximum volume thickness should be kept as small as possible, matched to the size of the vessel region of interest. A reduction of saturation can also be achieved by increasing the TR (see Fig. 5).

Larger vessel sections can be investigated by subdividing the volume of interest into several thin 3D slabs that are acquired sequentially [4]. One such multi-slab technique, which retains the advantages of 3D TOF yet has reduced saturation effects like 2D TOF, is called multiple overlapping thin slab acquisition (MOTSA). Generally, the chosen slab thickness has to be small enough to avoid saturation within the slabs. However, adjacent slabs must overlap by about 20 - 30% in order to compensate for signal attenuations arising at the slab edges due to non-rectangular excitation profiles. This results in compromised time efficiency and longer overall acquisition times.

The advantages and disadvantages of 2D and 3D TOF MRA are summarized in Table 3.

Table 4. Options to improve TOF angiography

Table 3. Comparison of 2D TOF and 3D TOF angiography

2D TOF

3D TOF

Strong inflow effect, minimal saturation

• sensitive even to slow flow (veins)

More saturation effects

• sensitive to rather fast flow (arteries)

Relatively poor signal-to-noise ratio

High signal-to-noise ratio

Short scan times

Poor background suppression

Relatively thick slices

• suitable for large vessels

Thin slices, allows isotropic voxels

• suitable for small vessels

Poor in-plane flow sensitivity

• for straight, unidirectional flow

Better than 2D TOF for tortuous vessels

Long echo times

Short echo times, less dephasing

Step artifacts at the vessel wall

Smoother vessel walls

Orientation of slices or volume perpendicular to flow direction

2D for slow flow, 3D for fast flow

3D multi-slab for larger vessel sections

Spatial presaturation to isolate arteries and veins

Use of minimum TE reduces signal loss due to spin dephasing

TONE pulse reduces saturation effects in 3D TOF

Magnetization transfer (MTC) and fat suppression improve vessel contrast

Frequently, 2D TOF MRA is favored for imaging veins because of the high sensitivity to slow flow. 3D TOF MRA, on the other hand, is more appropriate for fast arterial flow and for those cases in which high spatial resolution is required.

Was this article helpful?

0 0
Your Heart and Nutrition

Your Heart and Nutrition

Prevention is better than a cure. Learn how to cherish your heart by taking the necessary means to keep it pumping healthily and steadily through your life.

Get My Free Ebook


Responses

  • Karolin
    What is TOF 3D multi slab?
    8 years ago

Post a comment